The goal of this project is to elucidate the mechanism of epigenetic programming in germ cells, especially focusing on the roles of DNA damage response pathways in sex chromosome inactivation. Germ cells are capable of unique epigenetic programming which is required for sexual reproduction. A better understanding of the epigenetic program in germ cells will illuminate various reproductive issues underlying infertility and birth defects. When germ cells undergo male meiosis to generate haploid sperm, X and Y chromosomes go through a distinct epigenetic program different from autosomes. At the pachytene stage of meiosis, the genes on the X and Y are epigenetically silenced in a process called meiotic sex chromosome inactivation (MSCI). The entire X and Y chromosomes form a chromatin domain, known as the XY body, which is distinct from autosome regions. The XY body is marked by various chromosome-wide epigenetic modifications, which presumably maintain MSCI. My postdoctoral work revealed that sex chromosome inactivation is maintained even after meiosis, and implicated epigenetic inheritance in spermiogenesis and the embryonic development of the next generation. In this proposal, we aim to dissect the molecular basis of epigenetic silencing of sex chromosomes. An intriguing link between epigenetic silencing of sex chromosomes and components involved in DNA damage response (DDR) pathways has been suggested. Cytological evidence shows that various components involved in DDR pathways accumulate on the X and Y at the onset of MSCI. Based on our preliminary studies using mouse models defective for DDR pathways, we hypothesize that DDR pathways are adapted to initiate and maintain chromosome-wide silencing of sex chromosomes in germ cells. We will investigate the role of DDR pathways in MSCI by: (Aim 1) determining how chromosome-wide silencing is initiated; (Aim 2) dissecting the genetic pathways that coordinate potential downstream pathways; and (Aim 3) determining how epigenetic modifications on sex chromosomes are established to maintain inactivation. The proposed study in germ cells will potentially reveal a link between DDR pathways and epigenetic programming that can be generalized to somatic cells.